Here's how Matt has set up his first few tiles:
Starting in the top left and then working to the right and down I have the following on the Lumia 900:
Phone: Yes, I still use my device for calls and in a quick two taps I am calling my most recent people on the Lumia 900.
Family group: Windows Phone 7.5 added the ability to create groups and this function itself is a MAJOR selling point for the OS to me. I have my immediate and close family members all in this group where I can quickly send them all an email or text, view what is new on their Facebook page, and see what new photos they have shared. Once you set these up, I think you will find them essential as well.
Messaging: Another awesome integrated functionality of Windows Phone is the ability to have both text messaging and Facebook Chat integrated into the Messaging app so you can keep the conversation going in a method that you choose.
Linked Inbox: Here I go again with another integrated functionality. I have an Exchange account for work and a Gmail account for personal and 2nd job communications so prefer to have an inbox that brings this all together in one tile that shows me how many emails have arrived since I last read them.
You can read more about how Matt Miller (Palmsolo on Twitter) has his Lumia 900 set up via the source link below.